In various mechanical devices, such as automatic transmissions, a fluid is circulated as a coolant, a lubricant, and to transmit force, e.g., as a hydraulic fluid to apply clutches. Typically, the fluid contains a base oil and additives to provide the fluid with certain performance properties for a given fluid application. During circulation in an automatic transmission, fluid from a sump is pressurized by a pump, circulated through tubes, valves, and orifices, etc., and then allowed to drain back into the sump. During circulation, the fluid is agitated and exposed to air, and as a result, air becomes entrained in the fluid.
Air entrainment is undesirable for several reasons. First, entrained air can chemically degrade the fluid, causing it to lose its desirable functional characteristics. Entrained air contains water vapor, nitrogen, and oxygen, which can degrade the carbon chain molecules of the base oil and the additives. For example, oxygen can change the base oil into a hydroperoxide, an alcohol, or an acid.
Second, entrained air is compressible; thus, the fluid with entrained air may not respond as desired when transmitting a force as a hydraulic fluid due to natural frequency and bulk modulus changes. Third, entrained air will also compromise the fluid film strength thus reducing the fluid's lubricity.
Due to air entrainment's negative effects, it is desirable to utilize fluids in automatic transmissions and other mechanical devices that are formulated to meet predetermined air entrainment criteria, e.g., the amount of air entrained in the fluids must be less than a predetermined amount after significant agitation and exposure to air. The prior art includes various air entrainment test apparatuses and methods used to determine a fluid's suitability in a particular application. Typically, the prior art apparatuses and methods involve agitating the fluid to cause exposure to air, and then determining the amount of air entrainment by measuring the volume of the fluid or attempting to separate the entrained air from the fluid. Such techniques are time consuming and severely limit the frequency of measurement. Prior art air entrainment test apparatuses and methods are deficient in that they fail to, or are unable to, measure the transient air entrainment characteristics of a fluid such as rates of air entrainment or disentrainment after a change in the amount or intensity of agitation. For example, prior art test apparatuses cannot measure and calculate air entrainment data from the first few seconds following a change in agitation, nor during agitation.